Because of the line structure of the television picture such a television signal is essentially assembled from components which are spaced by the line frequency f.sub.h of, for example, 15,625 Hz. The brightness signal consists of such components which start at zero and which may reach to the resolution limit at for example 5 MHz. The two components u and v of the chrominance differential signal which correspond to the blue and the red colour difference signal are located at the line frequency distance above and below the chrominance signal subcarrier frequency f.sub.o. The u-components are located on either side of the chrominance signal subcarrier at a distance f.sub.h ; in contrast therewith the v-components are shifted by half the line frequency f.sub.h.
It is known to separate the u- and v-components by means of a decoding arrangement of a comb filter type. To this end the composite signal which contains at least the chrominance signal components is applied to a combining stage, on the one hand directly and on the other hand via a delay device. It is, for example, possible to obtain the u-chrominance signal in a substracting combining stage, when the chrominance signals are delayed in the delay device by a time period corresponding to 283.5 periods of the chrominance signal subcarrier f.sub.o and to obtain the v-chrominance signal in an adding combining stage. In a device which produces a time delay of 283.5 periods a total delay of 63.9433 .mu.s is obtained. It is alternatively possible to use a device which produces a delay of 284 chrominance signal subcarrier oscillations, which correspond to 64.056 .mu.s; the above-mentioned signals are then obtained at the other combining stage, that is to say the u-signal is obtained at the adder stage and the v-signal at the subtracting stage.
With a standardised PAL-colour television signal the subcarrier frequency is not a multiple of the line frequency, but it holds in accordance with the European standard that: EQU f.sub.o =283,75 f.sub.h +f.sub.v ( 1).
When the vertical deflection frequency f.sub.v is 25 Hz, then: EQU f.sub.o =283,7516 f.sub.h =4,43361875 MHz (2).
The maxima and minima of the transmission curves 1, shown in FIG. 1, which are formed by the comb filter properties are defined, in dependence on the frequency f, by the formula: EQU A.about..vertline. sin .pi..multidot.(283,5/f.sub.o).multidot.f.vertline.(3).
The maxima are located in the position where the sine is equal to (k+1/2).pi.;
the minima are found at k .pi., wherein k is an integer.
The frequency f.sub.x, at which the maximum of the transmission curve occurs, and the frequency f.sub.n, at which the minimum of the transmission curve occurs are found with the above-mentioned values for the chrominance signal subcarrier f.sub.o and the delay by 283.5 chrominance subcarrier periods at EQU f.sub.x =(k+1/2)(283.7516/283,5)f.sub.h and (4) EQU f.sub.n =k.multidot.(283,7516/283,5).multidot.f.sub.h, respectively (5)
From these formulae it appears that the period of the extremes of the transmission curve 1 do not accurately correspond with a muliple of the line frequency f.sub.h. So, when it is ensured, in accordance with the above-indicated customary dimensioning that the extreme values of the transmission curve 1 in the vicinity of the chrominance signal subcarrier correspond substantially exactly with the components of the colour difference signals u and v, a deviation occurs at a somewhat greater frequency distance. As far as the maxima are concerned this is hardly noticeable, as the tops of the sine waves change their value only little at small shifts. At the minima which correspond to the steep edges of the half sine waves the changes are more considerable, but these must be put up with.
FIG. 1 shows portions of the transmission curve along an abscissa, which is divided into sections and scaled in multiples of the frequency f, divided by the line frequency f.sub.h. The expanded curve 1 was obtained by means of a device producing a time delay of 283.5 periods of the chrominance subcarrier f.sub.o, the output signals of this device being combined with the undelayed signal in a subtracting stage. Above this curve there are shown by means of short upward lines on a horizontal line the u-frequency components of the chrominance signal, while the v-frequency components are represented by downward lines. These components are spaced by a frequency distance which is equal to the line frequency.
When the above-mentioned delayed and the undelayed signals are applied to an adder stage a transmission curve 2 which is defined by the formula: EQU A.about..vertline. cos .pi.283.5 f/f.sub.o .vertline.,
is obtained, which varies in accordance with the broken line curve shown in FIG. 1. It is shown that in the region of the chrominance subcarrier f.sub.o, the u-components are located at the maxima of the expanded curve 1 and the v-components at the maxima of the broken-line curves 2. For components which are further away from the chrominance sub-carrier, for example at 211 f/f.sub.h which corresponds to a modulation frequency of 1.14 MHz, there are small shifts with respect to the assigned maximum, which, however, have no effect on filtering of the said modulation components.
Each time the other modulation component is found on these comb filter transmission curves 1 and 2 in the vicinity of the chrominance subcarrier f.sub.o in the range of the peaks of minimum transmission, so that these components are removed by filtering to a considerable extent. For frequencies which are located at a greater distance, for example at the abscissa value 211, which corresponds to 1.14 MHz there is a certain shift of the peaks to the left, so to the lower frequency value, so that then the suppression of each time the other frequency component is not absolute. This is however put up with.
The components of the brightness signals correspond to the abscissa values shown in FIG. 1. These values are located at the edges of the sine and cosine tops, repsectively, of the transmission curves 1 and 2 in such manner that they are somewhat attenuated. As the higher frequency components of the brightness signal have only a rather low energy content and as they are often additionally reduced by an IF-drop in the vicinity of the chrominance subcarrier, the resultant disturbances may be put up with.
Experiments performed showed however that for the circuit arrangement described so far, which produces output signals corresponding to the transmission curves 1 and 2, the minima of the transmission curve 2 move at lower frequencies away from the brightness components and that in a corresponding manner the maxima of the transmission curve 2 move towards the brightness components. This means that in the separated v-chrominance signal the brightness components of, for example, 3.5 MHz and less come through stronger than the brightness components in the range of the chrominance subcarrier frequency f.sub.o of 4.43 Mhz. As furthermore the lower-freqency brightness components have also a higher energy content, they may produce clearly perceptible disturbances in the chrominance signal.
The invention has for its object to provide a circuit arrangement of the type defined in the preamble of such a construction that in the two separated chrominance signal components the lower-frequency brightness signal contents are suppressed better than in the vicinity of the chrominance signal subcarrier.